The present invention relates to a high reliability thin film semiconductor integrated circuit for preventing deterioration of an N-channel type thin film transistor (TFT).
As shown in FIG. 4, an invertor circuit is constructed by connecting the drain electrode of a P-channel type thin film transistor (TIT) 401 with the drain electrode of an N-channel type TFT 402. In this state, a large current flows into the drain electrode of the N-channel type TFT 402. As shown in Jig.5, if the drain voltage in the N-channel type TFT 402 is high, an electron within the gate electrode of the N-channel type TIT 402 is trapped in an oxide film as an insulating film near a drain region, so that a weak P-type region is formed in a interface portion between the drain region and a channel forming region. This prevents a drain current in the N-channel type TFT 402. Therefore, it is required that V.sub.DS is increased in comparison with a normal case, as shown in FIG. 6 and the thickness of a channel forming region of the N-channel type TFT 402 is increased, to reduce influence by the weak P-type region. As a result, characteristics of an N-channel type TIT change and deteriorate easily in comparison with a P-channel type so that deterioration of the characteristics reduces the reliability of a thin film semiconductor integrated circuit. This is the same in another basic circuit, for example, a NAND circuit. That is, as similar to an invertor circuit, an earthed N-channel type TFT is deteriorated easily.
As described above, when a drain voltage is high, a strong electric field (having a high strength) is generated near a drain region and therefore a weak P-type region is formed within a channel forming region, so that a drain current is prevented. Accordingly, characteristics of an N-channel type TFT deteriorate easily in comparison with that of a P-channel type TFT.